The present invention relates to electro-hydraulic braking systems and is concerned in particular with a method for controlling the speed of a pump motor within a pump supplying such a braking system.
Electro-hydraulic braking systems for motor vehicles are known which comprise a brake pedal, a braking device connected to at least one vehicle wheel which is capable of being brought into communication with an electronically controlled valve arrangement in order to apply hydraulic fluid under pressure to the braking device, a hydraulic pump driven by an electric motor, and a high pressure hydraulic pressure accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking device via the electronically controlled valve arrangement in order to apply hydraulic fluid under pressure to the braking device in so called xe2x80x9cbrake by wirexe2x80x9d mode in proportion to the driver""s braking demand as sensed at the brake pedal.
The known systems are commonly controlled by an electronic control unit (ECU) which performs high-level algorithms which control vehicle dynamic intervention functions, such as ABS, Traction Control TC and Vehicle Stability Control VSC. The electronic control unit controls among others the hydraulic pump to keep the pressure in the hydraulic pressure accumulator within specified limits.
When the brakes are actuated, and particularly when vehicle dynamic intervention is active, the pressure in the hydraulic pressure reservoir falls and must be increased by running the hydraulic pump. The rate at which the pump has to be run is dependent on the fluid consumption of the system, which is dependent on the vehicle dynamic functions (ABS, TC, VSC and the like) active at the time. However, the hydraulic pump used to pump up the high pressure accumulator is noisy and the noise increases with the speed at which it is run.
The known systems have the following disadvantage.
Pump flow rates needed during normal braking scenarios are relatively low because the time interval between successive brake applications allows the accumulator to be recharged slowly. However, the pump must sometimes deliver high flow rates at low motor voltage (9v) in order to satisfy the requirements of vehicle dynamics systems such as ABS, and this determines the size of the pump and motor. As a consequence the pump is too noisy when the supply voltage is in the normal (13v) range because the operating speed, and therefore also delivery flow, increases further.
This excessive pump noise is also unnecessarily generated in the known systems when the brakes are repeatedly xe2x80x9cpumpedxe2x80x9d with the vehicle stationary, e.g. during xe2x80x9cshowroomxe2x80x9d assessments.
It is an object of the present invention to reduce the incidence of excessive pump noise in situations where high pump speeds are not really necessary.
In accordance with the present invention there is provided a vehicle braking system of the type in which a stored volume of a fluid is used to apply the wheel brakes of the vehicle under the control of an electronic control unit, the stored volume being held in an accumulator arranged to be replenished by pumping additional working fluid using a pump driven by an electric motor, wherein the speed of the pump electric motor is arranged to be controlled in dependence upon whether or not any vehicle dynamic intervention, such as ABS, TC and VSC, is active.
If it is deduced that one or more vehicle dynamic functions is active then the pump can be run at high/full, and relatively noisy, speed. On the other hand, if it is concluded that only base braking is being applied with no vehicle dynamic function being active, then the pump can be arranged to be operated at a relatively slower and hence quieter speed.
A preferred feature of the present invention is to provide a system for controlling the hydraulic pump motor such that it is run at full speed only when the reservoir pressure is substantially reduced and the vehicle is moving or at any time when demand is increased by the activation of vehicle dynamic intervention.
A specific embodiment of a braking system according to the present invention will now be described, by way of example only, with reference to the accompanying drawings.